1. Field of the Invention
The present invention relates to a technology for developing a latent image formed on an image carrier with a single-component developer.
2. Description of the Related Art
A tandem-type image forming apparatus forms developer images in different colors on a photosensitive element in a plurality of process cartridges, transfers the developer images onto an intermediate transfer unit in a superimposed manner, and transfers the superimposed image onto a transfer sheet, thereby forming a color image.
Each of the process cartridge includes, as a unit, the photosensitive element on which a latent image is formed; a roller charging device that charges a surface of the photosensitive element; a developing unit that develops a latent image on the surface of a photosensitive element with powdery developer, thereby forming the developer image; and a cleaning device that cleans residual developers remained on the photosensitive element after the transfer.
The developing unit includes a developer carrier that is in contact with the photosensitive element and that supplies the developer onto the photosensitive element; a developer supplying member and a layer shaping member that are arranged in contact with the developer carrier; and a developer housing unit that stores therein the developer. The intermediate transfer unit is formed of a belt. The process cartridges for each color are arranged in parallel and facing the intermediate transfer belt.
The developing unit charges the developer, and conveys the charged developer by the developer carrier passing through the layer shaping member that is charged. When the developer passes through the layer shaping member, an amount of the conveyed developer is regulated, and the conveyed developer is further charged. However, if a polarity of the voltage applied to the layer shaping member and a polarity of charged developer are opposite, normally the charged developer that is to be conveyed by the developer carrier scatters and adheres to the layer shaping member.
In other words, when providing an electric potential difference between the developer carrier and the layer shaping member that shapes the developer on the developer carrier into a layer, the developer that is shaped into the layer on the developer carrier scatters and accumulates on the layer shaping member, depending on a polarity of the charged developer or a charging distribution. The developer accumulated on the layer shaping member falls due to a vibration and the like of the image forming apparatus, which causes image failures or image degradation.
When a polarity of the voltage applied to the layer shaping member and a polarity of the charged developer are the same, a tiny amount of developer that is charged to a polarity opposite to the developer that passed through the layer shaping member scatters (flies) to the layer shaping member.
A typical way to solve the above-described problems is to set the voltage to be applied low, by making the polarity of the voltage applied to the layer shaping member and the polarity of the charged developer identical.
However, when setting the applied voltage low, although the problem of the developer scattering to the layer shaping member can be prevented, it is impossible to apply a voltage strong enough to improve the charging performance of the developer, which is a primary purpose.
Japanese Patent Application Laid-open No. H8-254894 discloses a method of stabilizing charging performance of the developer and removing the developer adhering to the layer shaping member by arranging a rotating member, serving as the layer shaping member for the developer. A re-charging member for the developer is in press contact with the rotating layer shaping member; therefore, the developer adhered to the layer shaping member is charged. In this way, the developer returns to the developer carrier in an electrostatic state.
Japanese Patent Application Laid-open No. H8-202128 discloses a method of reducing the developer adhered on a base surface of an image carrier, while improving supply performance of the developer, by setting voltages applied to the layer shaping member, a developer supplying member, and a developer carrier and setting polarities thereof in conforming with a polarity of a charged developer.
With the method disclosed in Japanese Patent Application Laid-open No. H8-254894, a high voltage can be applied; however, the configuration is complicated because the layer shaping member is formed of the rotational member. This causes problems such as complicated installation and an increase of the number of types of the required materials and the number of the required components.
With the method disclosed in Japanese Patent Application Laid-open No. H8-202128, the voltages and the polarities in conforming with the polarity of the charged developer are set; however, sufficient voltages cannot be applied.